Soil Acidity, pH, and Liming

One of the most important aspects of nutrient management is maintaining proper soil pH, a measure of soil acidity. A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is alkaline. Most New England soils are naturally acidic and need to be limed periodically to keep the pH in the range of 6.5 to 6.8 for most vegetable crops. Scab-susceptible potato varieties are an exception, but some lime may still be needed to maintain the recommended pH of 5.0-5.2. When the soil is acidic, the plant availability of nitrogen (N), phosphorus (P), and potassium (K) is reduced and there are usually low amounts of calcium (Ca) and magnesium (Mg) in the soil. In contrast, most micronutrients are more soluble and are therefore more available to plants. Under very acidic conditions aluminum (Al), iron (Fe), and manganese (Mn) may be so soluble they can reach toxic levels. Soil acidity also influences soil microbes, which decompose organic matter and recycle crop nutrients. For example, when soil pH is low (below 6.0), bacterial activity is reduced and fungal activity increases. Acidic soil conditions also reduce the effectiveness of some pesticides.

The most effective way to manage soil acidity is to apply agricultural limestone. The quantity of lime required is determined by the target pH (based on crops to be grown) and the soil's buffering capacity, measured in a soil test. Buffering capacity refers to the soil’s tendency to resist change in pH. Soil pH is only a measure of active acidity, the concentration of hydrogen ions (H+) in soil solution. When lime is added to a soil, active acidity is neutralized by chemical reactions that remove hydrogen ions from the soil solution. However, there are also acidic cations (H+ and Al+) adsorbed on soil colloids (see CEC, previous section) that can be released into the soil solution to replace those neutralized by the lime. This is called reserve acidity. Clays and soils high in organic matter have the potential for large amounts of reserve acidity. These soils are said to be well-buffered. To effectively raise the soil pH, both active and reserve acidity must be neutralized. Soil test labs determine buffering capacity and lime requirement by measuring or estimating the reserve acidity. 

The neutralizing power of lime is determined by its calcium carbonate equivalence. Suppliers can tell you the calcium carbonate equivalence of the lime you are purchasing. Recommendations are based on an assumed calcium carbonate equivalence of 100. If your lime is lower than 100, you will need to apply more than the recommended amount, and if it is higher, you will need less. To determine the amount of lime to apply, divide the recommended amount by the percent calcium carbonate equivalence of your lime and multiply by 100. Wood ash is another amendment that may be used to manage soil acidity. The calcium carbonate equivalence of wood ash is typically around 50%, but it can vary widely. If purchasing wood ash from a supplier, they will provide a recent analysis. Otherwise the wood ash should be submitted to a lab offering lime analysis to determine the calcium carbonate equivalence.

The speed with which lime reacts in the soil is dependent on particle size and distribution in the soil. To determine fineness, lime particles are passed through sieves of various mesh sizes. A U.S. Standard 10-mesh sieve has 100 openings per square inch while a 100-mesh sieve has 10,000 openings per square inch. Lime particles that pass through a 100-mesh sieve are very fine and will dissolve and react rapidly (within a few weeks). Coarser material in the 20-30-mesh range will react over a longer period, one to two years or more. Agricultural ground limestone contains both coarse and fine particles. About half of a typical ground limestone consists of particles fine enough to react within a few months, but to be certain you should obtain a physical analysis from your supplier. Super fine or pulverized lime is sometimes used for a “quick fix” because all of the particles are fine enough to react rapidly.

Lime will react most rapidly if it is thoroughly incorporated to achieve intimate contact with soil particles. This is best accomplished when lime is applied to a fairly dry soil and disked in (preferably twice). When spread on a damp soil, lime tends to cake up and doesn’t mix well. A moldboard plow has little mixing action, therefore, disking is preferred.

Besides neutralizing acidity and raising soil pH, lime is also an important source of Ca and Mg for crop nutrition. It is important to select liming materials based on Ca and Mg soil content with the aim of achieving sufficient levels of each for crop nutrition. If the Mg level is low, a dolomitic lime (high magnesium lime) should be used; if Ca is below optimum, a calcitic (low-Mg lime) should be used.